Photopolymers are used to make either positive-working or negative-working patterns on various substrates. Solvent-developable photopolymers are a particular class of photopolymers that have the ability to either become more soluble, or less soluble, when exposed to light of a particular wavelength. Positive photopolymers become more soluble, and negative photopolymers become less soluble in a developer solution. When exposed through a patterned mask, after development of the resist with a solvent, a like or opposite pattern of the resist remains on the substrate. This developed or patterned photopolymer layer can then be used to prevent the action of a corrosive liquid or etchant (or etch gas when plasma etching is employed) from reacting with and removing portions of the substrate not protected by the photopolymer.
Photopolymers, or photoresists, have long been used in the semiconductor industry directly on semiconductor wafers to etch various features into the substrate. Resists are also used to make printed circuit boards wherein a conductive metal is deposited in photoresist openings to form printed circuits on the board substrate.
In the printing plate art, resists are also used to form an image on a substrate, which is generally a metal plate. Ink is applied to the plate having a patterned photoresist layer thereon, and the ink can be transferred to another medium, such as paper. This process is well known.
Recently, an improved printing plate technique has been developed that uses a bimetal, usually copper clad aluminum, for the printing plate. In order to make this plate, a photoresist is applied over the copper, and exposed to light of a desired frequency. Generally the photoresist is covered with a patterned film emulsion or mask so that only certain areas of the photoresist are exposed, as shown in FIG. 1A, which illustrates a dot pattern. The photoresist is hardened or cross linked by the light, the film emulsion or mask is removed, and the plate is developed to solubilize the unexposed regions, as shown in FIG. 1B. The substrate is then etched, whereupon the exposed copper layer is removed, as shown in FIG. 1C. Ink is applied to the copper remaining on the aluminum plate in a press, and thereafter transferred to another substrate. The inked image is shown in FIG. 1D.
With the coming of computer graphics and patterns available in digital form, the problems of direct conversion of such patterns and information to a photoresist covered printing plate are being addressed. This can be done using computer control of a laser light source to transfer the image information to a printing plate. A suitable laser scans across the plate, and the computer turns the laser on and off to pattern expose the photoresist. This eliminates the step of making a mask of the desired pattern, and, because the laser light can be finely controlled, an exact amount and frequency of light exposure of the photoresist and improved accuracy of the pattern can be obtained.
The photoresist for such a system is important. The photoresist exposed to light must change its solubility between the exposed and non-exposed regions; it must be developable with standard solvents; and the insoluble portions must be insoluble enough so that they remain insoluble for a time sufficient to ensure that the soluble portions are completely removed during development, but that at least some thickness of the insoluble portions of the photoresist remain after development. Further, a good photoresist for printing plates or other uses, such as for making printed circuit boards, must be able to withstand attack by the etch solution used to etch the exposed portions of the substrate. These requirements are not trivial.
Thus it would be highly desirable to develop photoresist formulations that moot the requirements discussed above so that very exact pattern replication of images from a computer controlled laser directly to a metal substrate can be obtained.